Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons, carbon monoxide, nitrogen oxides (“NOx”), sulfur oxides, and particulate matter. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Many different techniques have been applied to exhaust systems to clean the exhaust gas before it passes to atmosphere.
One such technique utilized to clean exhaust gas is the NOx trap (or “NOx adsorber catalyst”). NOx traps are devices that adsorb NOx under lean exhaust conditions, release the adsorbed NOx under rich conditions, and reduce the released NOx to form N2. A NOx trap typically includes a NOx adsorbent for the storage of NOx and an oxidation/reduction catalyst.
The NOx adsorbent component is typically an alkaline earth metal (such as Ba, Ca, Sr, and Mg), an alkali metal (such as K, Na, Li, and Cs), a rare earth metal (such as La, Y, Pr, and Nd), or combinations thereof. These metals are typically found in the form of oxides. The oxidation/reduction catalyst is typically one or more noble metals, preferably platinum, palladium, and/or rhodium. Typically, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. The oxidation/reduction catalyst and the NOx adsorbent are typically loaded on a support material such as an inorganic oxide for use in the exhaust system.
The NOx trap performs three functions. First, nitric oxide reacts with oxygen to produce NO2 in the presence of the oxidation catalyst. Second, the NO2 is adsorbed by the NOx adsorbent in the form of an inorganic nitrate (for example, BaO or BaCO3 is converted to Ba(NO3)2 on the NOx adsorbent). Lastly, when the engine runs under rich conditions, the stored inorganic nitrates decompose to form NO or NO2 which are then reduced to form N2 by reaction with carbon monoxide, hydrogen and/or hydrocarbons in the presence of the reduction catalyst. Typically, the nitrogen oxides are converted to nitrogen, carbon dioxide and water in the presence of heat, carbon monoxide and hydrocarbons in the exhaust stream.
Unfortunately, the presence of sulfur compounds in the diesel or gasoline fuel is detrimental to NOx traps since the oxidation of sulfur compounds leads to sulfur oxides in the exhaust gas. In the NOx trap, sulfur dioxide is oxidized to sulfur trioxide over the oxidation catalyst and the NOx adsorbent reacts with the sulfur trioxide to produce surface sulfates (e.g., barium oxide or barium carbonate reacts with sulfur trioxide to form barium sulfate). These sulfates are more stable than the nitrates and require higher temperatures (>650° C.) to desulfate. However, the extreme conditions required for desulfation and NOx trap regeneration can lead to longer term deactivation of the NOx trap and may lead to shortened NOx trap life.
U.S. Appl. Pub. No. 2010/0215557 describes an aging-resistant three way catalyst for reducing NOx in exhaust gases. The three way catalyst preferably comprises three layers: (1) a bottom “etch coat” layer containing no platinum group metals; (2) a middle (first catalytic) layer comprising Pd on a ceria-free oxygen storage component and Pt on a refractory metal oxide; and (3) a second catalytic layer formed on the first catalytic layer, comprising Pt on an oxygen storage component and Rh on zirconia-coated or yttria-coated alumina. U.S. Appl. Pub. No. 2010/0215557 does not describe the effects of sulfur on its catalyst.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems. We have discovered a new NOx trap that is less prone to deactivation over numerous desulfation/NOx trap regeneration cycles.